When a senator comes up with a probation program to redeem criminals in the USA, the Supreme Court offers him a chance to prove the feasibility of his idea... by trying it out on the worst Old West villains ever: the Daltons. As usual, it's up to Lucky Luke to keep an eye on the terrible four during their month-long trial period. If they can refrain from any crime during that time, they will be set free as good citizens...
